1. Field of the Invention
This invention relates to a stabilized antenna system on a moving platform like a ship to be used for satellite communications or for receiving the satellite broadcasting signal, and more particularly to a stabilized antenna system having a function to stabilize an array antenna against roll and pitch of such moving platform.
2. Description of the Related Art
Heretofore, directional antennas such as parabolic reflector antennas have been used for satellite communications. Historically, the maritime satellite communications was started in 1976 by using the MARISAT system. It was handed over in 1982 to the internationally organized INMARSAT system and has been in operation since then.
According to the technical requirements document for the standard-A ship earth station in the present INMARSAT system as of June 1987, the ship earth station should have G/T of -4 dBK at least. To meet this requirement, a parabolic reflector antenna should be about 0.8 meters (or more) in diameter, for example.
Further, a radome is necessary to make the parabolic reflector antenna resistant to rainwater and rough weather. Such radome should be about 1.2 meters in diameter for the parabolic reflector antenna of 0.8 meters in diameter. The radome is a dome-shaped housing made of material which can pass the microwaves (of approximately 1.5 GHz) for the satellite communications. Generally, FRP (Fibre Reinforced Plastics) is used for the radome. The radome is usually mounted on a radome base and the radome base has an access hutch to facilitate maintenance and repair work.
A stabilized antenna system has been known as a system described as above. This antenna system has a stabilization function as well as a satellite tracking function.
The antenna should be steered so that the antenna system installed on a moving platform such as a ship can well receive radio waves from the satellite. To track the satellite under roll and pitch motions, the antenna should be stabilized by mechanical or electronical means. A variety of technologies have been developed to steer the antenna to track the satellite under roll and pitch.
Sometimes the parabolic reflector antenna is steered by an antenna mount with three mechanical axes, such as an AZ-EL-XEL (Azimuth-Elevation-Cross Elevation) mount, for example.
An AZ axis is for steering the antenna in azimuth. An EL axis is for steering the antenna in elevation. Further, an XEL axis is perpendicular to the EL axis.
In the 3-axis antenna mount, when all the three axes are mechanical, the entire antenna mount tends to become heavy, large and complicated. To overcome such inconvenience, an antenna mount having two mechanical axes has been proposed.
Examples of such two-axis antenna mounts, AZ-EL mounts, are disclosed in "Control Method of 2-Axis Az-El Antenna Mount," by Yuki, et al., Electronic Communications Society, SANE83-53, page 1-6, and "Development of a Compact Antenna System for the INMARSAT standard B SES in Maritime Satellite Communications," by Shiokawa, et al., Electronic Communications Society, SANE 84-19, page 17-24.
However, an AZ-EL mount has a problem of a singular point in the direction for the zenith.
To cope with such singular point, each axis of the AZ-EL mount should be controlled by a highly sophisticated wideband servo control means. Such a wideband servo control means tends to be costly. Even when these sophisticated measures are taken, there are data showing that a tracking error of about 10.degree. exists in the vicinity of the singular point.
To overcome the foregoing inconveniences, there is currently known an antenna system which steers the beam electronically. Such electronic steering is realized by a so-called phased array antenna.
An antenna system with phased array antenna is disclosed in "Phased Array Antenna for MARISAT Communications," Folkebolinder, Microwave Journal, 1978, 12, pp 39-42. This system includes an AZ axis for mechanical steering in azimuth and two planar array antennas including a plurality of antenna elements arranged on two panels and variable phase shifters for controlling the beam directivity thereof. (For the simplicity, variable phase shifter may be described as "phase shifter".)
Specifically, the phase shifters are connected to the individual antenna elements. The phase shifters control the amount of phase of signals related to the antenna elements. By controlling the amount of phase shift, beam directivity of the antenna can be varied as desired.
However, even when the electronic steering is performed as described above, the phase shifters should be mounted for the respective antenna elements one to one basis, so that the overall antenna will become large, complicated and expensive. Therefore, application of the foregoing antenna system has been somewhat limited.
Antenna systems are disclosed in Japanese Patent Laid Open Publication No. SHO 51-110950 to cope with the above-described inconveniences. The publication describes a plurality of array antennas to be installed on ships for the maritime satellite communications. One of the antenna systems comprises AZ and EL axes for mechanical steering to control beam patterns by combining outputs from a plurality of array antennas. This system is simplified, small, less expensive, and easy to maintain.
A further example of the antenna system allowing electronic steering is disclosed in the co-pending "Method of Antenna Stabilization and Stabilized Antenna System", Japanese Patent Application No. HEI 2-339317. The citation relates to an X1-Y-X2 antenna mount without AZ and EL axes. The X1 and Y axes are mechanically steered, and the X2 axis is electronically steered. Therefore, the whole antenna system is simplified and less expensive.
However, in any of the above-cited examples, the array antennas have antenna elements arranged in the shape of lattice. In the so-called AZ-EL-XEL mount, if the AZ and El axes were mechanical and if the XEL (cross-elevation) axis were electronical, there would be an inconvenience that the phase shifters would have to control a large angular area, because a horizontal distance between the adjacent antenna elements would be relatively large as described later.
FIG. 18 of the accompanying drawings shows an array antenna with a (2, 2, 2) element arrangement.
As shown in FIG. 18, antenna elements 10 are arranged in the shape of lattice on a base plate 12. The horizontal distance between the two adjacent antenna elements 10 is expressed by dx, and the vertical distance is expressed by dy. Theoretically, a diameter of each antenna element is about .lambda./2 (.lambda.: wavelength). In the illustrated arrangement, both dx and dy should be .lambda./2 or more to prevent overlapping of the antenna elements 10.
FIG. 19 shows the configuration of the AZ-EL-XEL mount, which has AZ, EL and XEL axes. The AZ axis is steered to adjust the azimuth, and the EL axis is steered to adjust the elevation angle. The XEL axis is steered to adjust the cross-elevational angle in a plane parallel to the EL axis. If the AZ and EL axes were mechanically steered to angularly move the array antenna 10 and if signals received by the antenna elements 10 on the array antenna 12 were phase-shifted by a phase shifter to steer the beams around the XEL axis perpendicular to the EL axis, an AZ-EL-XEL mount which includes an electronically controlled XEL axis could be realized. For example, if the array antenna 12 were lengthwisely mounted in parallel to the EL axis and if one variable phase shifter were disposed for each pair of vertically aligned antenna elements, the antenna beam could be steered for the XEL axis by giving the phase shift commands to the phase shifters. In other words, the XEL axis could be electronically steered.
FIG. 20 shows radiation patterns of the array antenna 12 having the mechanically steered AZ-axis and EL-axis, and the electronical XEL-axis of FIG. 18.
In FIG. 20, the radiation pattern A0 is obtained when phase shift of the phase shifter is 0.degree. for each antenna element 10. The radiation pattern A1 is obtained when phase shift are plus/minus 90.degree. for the two antenna elements in the left/right columns, respectively and is 0.degree. for the two central antenna elements.
In these radiation patterns A0 and A1, a first sidelobe has peaks at positions deviating about plus/minus 45.degree. from the main lobe (beam). The peak of first sidelobe related to the radiation pattern A0 is about -13 dB for the peak of the main lobe, and the first peak of the first sidelobe related to the radiation pattern A1 is about -10 dB for the peak of the main lobe.
When such remarkable sidelobes appear, the antenna system decreases its efficiency and radiates radio waves in unnecessary directions, thereby possibly interfering other communication systems.
If an array antenna of the conventional lattice arrangement such as in FIG. 18 were adopted for an example antenna in the electronic XEL axis and the electronic XEL axis were inclined, the remarkable sidelobes should appear. In FIG. 18, the larger the phase shift of the phase shifter, the more remarkable sidelobes occur. In other words, the more the electronic XEL axis is inclined, the more remarkably the sidelobes occur. For example, minimum requirements of the roll angle and pitch angle for the INMARSAT-M Ship Earth Station are plus/minus 25.degree. and plus/minus 15.degree., respectively. (Reference will be made to "INMARSAT-M SYSTEM DEFINITION MANUAL (issue 2) MODULE 2 3.6.2.2 Recommended Environmental Conditions for Maritime Class MESs.) If the ship inclines together with the antenna system when a satellite as a tracking target exists in the direction along the bow and stern of the ship and near the zenith, the XEL axis should be inclined most extensively. In the above-described case, the antenna beam should cover at least a range of about plus/minus 25 degrees around the XEL axis. Unfortunately, if an antenna beam of the conventional lattice arrangement array antenna were inclined to cover the range of plus/minus 25 degrees around the XEL axis, the disadvantage of remarkable sidelobes would appear.